Feedback based process control programs are well known in the art. These programs may monitor the values of variable parameters and compare these values to variable parameter set points to determine error values associated with each variable parameter. The program may then adjust one or more output values seeking to change the value of the variable parameter and to reduce the error value toward zero.
Plotting the values of a variable parameter and the set point for the parameter over time illustrates the error correction response profile for the variable parameter. The rate and manner at which the control program reduces the error value of the variable parameter may influence the response profile. The program may change the rate of error correction by adjusting either the rate of integration or the amount of proportional correction or both.
The physical realities of the controlled process may make particular characteristics of a response profile more or less desirable. The degree to which any particular characteristic is desirable may change over time and may depend upon other aspects of the controlled process. A method to provide flexibility in the characteristics of the error correction response profile is therefore desirable.
Those of skill in the art know that the use of gain scheduling may provide increased flexibility in control programs. Control programs may use gain scheduling to alter the relationship between a second variable parameter and the set point for the first variable parameter depending upon the value of a third variable parameter.
Control programs may use the magnitude of the error value associated with a first variable parameter to schedule the gain that determines the rate of correction of the error value of the first variable parameter.
Using gain scheduling to adjust the relationship between a first variable and the set point for a second variable, or for adjusting the rate of error correction associated with a first variable based upon the magnitude of the error associated with the first variable may not provide sufficient flexibility in achieving the rate of response and the desired response profile characteristics in all circumstances.
The winding of web materials may benefit from flexibility in the rate of response and response profile characteristics. Web materials constitute a common element of daily life. Metal films, non woven substrates, and paper products exemplify these web materials. The commercial production of these and other web materials may require the winding of the web material around a spool into a roll. The web material of the wound roll may subsequently be otherwise processed. The uniformity of the winding of a roll may affect the ability to successfully process the material of a roll, and the quality of any subsequent product produced from the material of the roll. Processing rolls wound in a non-uniform manner may not be possible or these rolls may yield products of unsatisfactorily low quality.
In the winding process, the web material may pass through a nip point formed between the roll being wound and a support structure of the web such as a winding reel. The nip pressure of the winding process may affect the quality of the winding of a roll. The nip pressure refers to the force applied to the web as the web passes through the nip point. An excessive nip pressure may break or damage the web. An insufficient nip pressure may result in a wrinkled or folded web, or a loosely wound roll. A non-uniform nip pressure over the winding of the roll may yield a non-uniform roll.
A feedback control loop may control the magnitude of the nip pressure. Portions of the winding process may benefit from adjustable error correction response profiles. Rolls of material wound by the process may benefit from adjusting the nip pressure error correction response profile during the winding process.